The present invention relates to an accumulator for suppressing an abrupt rise in and accumulating the pressure of a fluid.
As is well known in the art, an automatic transmission for vehicles is constructed to change the transmission of a drive power in a gear train mechanism by engaging or releasing frictional engagement means such as a clutch or brake, and to change a gear ratio which may be set according to the output power of an engine and number of revolutions of an output shaft. The operations of the clutch or brake for switching that gear ratio is performed hydraulically, and shocks are caused if the hydraulic pressure rises abruptly for the gear shift. Generally in the prior art, therefore, in order to eliminate the gear shifting shocks, there is disposed upstream of the frictional engagement means an accumulator which can soften the abrupt rise or drop of the hydraulic pressure when the shift valve is switched.
One fundamental structure of the accumulator of the prior art is disclosed in Japanese Patent KOKAI No. 59 - 144801, for example. In this disclosure, a bottomed cylindrical piston is fitted slidable back and forth, while having its outer circumference sealed up with O-rings, in a cylinder formed in a valve body. A spring for acting against the hydraulic pressure is arranged to have its one end portion inserted in the piston so that the piston may be slowly moved while compressing the spring by that hydraulic pressure to soften the rise of the pressure.
In the accumulator having that fundamental structure, the sealing O-rings will establish a sliding resistance which will adversely affect the characteristics of the accumulator and accordingly the characteristics of the hydraulic circuit as a whole. For this reason, another spring is provided in the art to counteract the sliding resistance of the O-rings while the accumulator is operating.
In the accumulator of the prior art thus far described, the spring is retained in position by inserting it into a hollow portion of the piston. In order to improve the assemblability of the piston, it is preferable to set a gap between the outer circumference of the spring and the inner circumference of the piston thereby to bring the two members into the so-called "loosely fitted state". In this assembly, however, the spring is not fixed but allowed to move so that it has bad stability. Thus, the spring may go out of position to experience an interference with the piston such a frictional slide on the inner circumference of the piston. This raises a problem that the spring has its durability deteriorated. In order to solve this problem, it is conceivable to insert and retain the spring in close contact with the inner circumference of the hollow portion of the piston. With this structure, however, absence of any substantial gap between the spring and the piston will make it difficult to insert the spring snugly. Therefore, this structure encounters with a problem that the assemblability is inferior.
It is, therefore necessary for the structure of the prior art to set the difference between the external and internal radii and the spring and piston at such a value as to satisfy both the assemblability and stability of the spring. This necessity makes it difficult to manufacture and control the quality of the accumulator structure. Otherwise, another problem arises in that either the assemblability or stability of assembly has to sacrificed.
In the specification of U.S.P. No. 3,782,708, on the other hand, there is a structure for securing a coil spring on a plate. The plate has its one side punched to extrude a cylindrical nub, on the outer circumference of which is fitted one turn of one side of a coil spring. The nub is deformed outward to trap and secure that turn of the spring.
However, this structure restricts its spring securing member to the plate, which has to be formed with the punched hole. As a result, the spring securing structure cannot be used, as it is, for securing the so-called "block-shaped object" such as the piston of the aforementioned accumulator.